I found the following question and answer that solved the problem for me. It contains a list of replacements for the old applets called application indicators. Unfortunately not all of them are available for natty yet, but at least I got a very basic system load monitor (indicator-sysmonitor) and a weather indicator (indicator-weather) working.

thanks James for the formatting and the missing step (apt-add-repository).
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krumpelstiltskinApr 28 '11 at 18:09

Could you please tell me how i could get the RAM % instead of disk usage?
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neoMay 3 '11 at 16:13

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@neo: make a function that parses "/proc/meminfo" much like the one called "getInOut()" (the fields in meminfo are self explainatory). Then call your new function with makeSample(). If there's demand for this I'll write and post the code.
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krumpelstiltskinMay 4 '11 at 21:21

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@neo: i added the lines for memory usage to the post. if mem usage is all you want, I suggest you remove all the other lines from the script so python doesn't have to parse them.
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krumpelstiltskinMay 6 '11 at 20:19

+1 I've been using Conky for this exactly (as a system monitor applet replacement). It's extremely configurable, and it's likely that it will take some work to get the desired result. For some good-looking and interesting configs, I've been using one suggested on webupd8.org
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belacqApr 15 '11 at 3:27